Modem communications employs many types of radio frequency transmitters and receivers. One type of radio frequency receiver is a direct conversion receiver. In a direct conversion receiver a received signal is mixed directly to a zero-intermediate frequency (IF), wherein the data encoded in the received signal is directly demodulated. The advantages of a direct conversion receiver are that it requires less circuitry, requires less complex circuitry, and produces a cheaper receiver due to the need for fewer components.
FIG. 1 shows a desired channel B that exists in the frequency spectrum. The desired channel B may exist along with adjacent channels A and C. It is desired that a receiver be able to receive and demodulate the information in signal B, but yet without receiving or demodulating any signal from channels A and C.
FIG. 2 shows a down-conversion of the desired channel B to a zero-IF frequency, wherein the down-converted channel B is now centered at zero Hertz (i.e., the carrier wave has been completely removed). The adjacent channel A and the adjacent channel C can be removed by a low pass filtering (represented by the dashed line). This leaves the channel B to be demodulated. Advantageously, the zero-IF down-conversion and low pass filtering eliminate any positive or negative image frequencies.
However, a problem with a direct conversion receiver of the prior art is that several anomalies may occur in the signal due to the down-conversion process. One anomaly is a DC bias in the down-converted signal. Another anomaly is an amplitude modulation (AM) notching effect that is created whenever the AM carrier is removed. Yet another anomaly is an AM sideband notching that occurs when any sideband energy of the AM signal falls at zero Hertz and the In-phase and Quadrature (I/Q) channel DC offset errors have been measured and removed. In addition, other down-conversion anomalies may include frequency modulation (FM) notching, microphonics (i.e., errors induced due to the physical motion of the receiver), synthesizer 1/f noise, and phase and gain errors.
FIG. 3 is a block diagram of a quadrature direct conversion receiver. The receiver includes two local oscillators (LO) 303i and 303q for down-converting the I/Q components of a received signal. These two LOs 303i and 303q are generally depicted as a single analog mixer. Phase and gain errors in such a receiver are commonly a combination of phase and gain errors between the I and Q components in the LOs 303i and 303q respectively and a mismatch in the analog baseband circuitry (the phase and gain errors are referenced relative to the I channel). In the mixer portion of the receiver, the theta (θ) term in the Q channel is a phase error term that is present due to the mismatch in time delay between the two LOs 303i and 303q (i.e., they may not be perfectly ninety degrees apart). The GO term in the Q channel is a gain error term that is present due to the power mismatch between the two LOs (i.e., the power in each oscillator is not identical). There is also a phase and gain error contribution from the baseband analog portion of the receiver. The low pass filters (LPFs) 305i and 305q generally cannot be perfectly matched, and will contribute a gain error term and a phase error term. Moreover, the attenuators 308i and 308q and the baseband gain 309i and 309q are not perfectly matched, and will also create a gain error term. As a result, the phase error term will be frequency dependent and will change across frequency. Furthermore, the phase and gain errors will change across temperature. Therefore, the phase and gain errors may be continually changing, and need to be continuously calculated in order to be corrected.
Attempts have been made in the prior art to address these anomalies by using a phase/gain error correction algorithm in conjunction with a zero-IF frequency down-conversion. One such phase/gain error detection and correction method employs rotating vectors and is disclosed in Loper, U.S. Pat. No. 5,230,099, and also in Loper et al., U.S. Pat. No. 5,604,929, both of which are incorporated herein by reference. However, one significant drawback of a zero-IF conversion is that phase/gain error correction methods known in the art cannot be used at zero Hertz. The rotating vector error correction method performs well when calibrating the phase and gain errors with an adjacent channel interferer present, but a new problem is created when there is an adjacent channel spectrally coherent interferer. The rotating vector phase and gain correction cannot perform phase and gain error calculations and corrections if there is an adjacent channel interferer that is a coherent spectral image of the desired channel. This arises because a coherent interferer's I/Q vector rotates in the opposite direction from the desired channel's I/Q vector and adds to the desired channel's image, and the desired channel adds to the interferer's image, creating a perceived receiver phase and gain error that is incorrect. In addition, a coherent interferer notching effect can be created. The AM notching effect, the AM sideband notching effect, and the FM notching effect phenomena's can be created at the IF when the interferer has a discrete spectral line that is a frequency coherent image of a spectral line in the desired band. The adjacent channel interferer does not need to be a perfectly (phase and frequency) coherent image of the desired channel signal in order to create problems for the rotating vector phase and gain error correction. It only needs to be frequency coherent, because this phenomena is primarily a discrete spectral line issue.
The phase/gain error correction problem has been addressed in the prior art by tuning the LO off of the zero-IF frequency to produce essentially a baseband signal. However, as a consequence, an adjacent channel interferer may now be a positive/negative image frequency, and notching can occur at this IF.
FIG. 4 shows a down-conversion of the desired channel B to a baseband positive IF frequency that is not centered at zero Hertz. Typically, a down-converted baseband signal extends from zero Hertz to the upper limit of the radio frequency (RF) channel. The adjacent channel C may be removed through a low pass filter (again represented by a dashed line). Because adjacent channel A has been down-converted to a negative image frequency, it may still exist and may still interfere with the desired channel B.
FIG. 5 shows a down-conversion of the desired channel B to a baseband negative IF frequency. The adjacent channel A may be removed by a low pass filtering (again represented by a dashed line). However, the adjacent channel interferer C remains. It should be understood that when channel A or C remains, there remains a potential for this adjacent channel to become an adjacent channel interferer and interfere with the signal of channel B. The adjacent channel interferer may contribute to the aforementioned down-conversion anomalies.
Phase and gain errors in a quadrature direct conversion non-zero-IF receiver create an image frequency that adds distortion to the baseband demodulated AM/FM or I/Q. The result is that a direct conversion receiver according to the prior art is subject to an either/or situation. A great advantage of a quadrature zero-IF direct conversion is that no positive or negative image frequencies are created if the carrier is mixed down to exactly zero Hertz. However, the known phase/gain error correction methods do not function for a zero-IF system.
What is needed, therefore, are improvements in direct conversion receivers in order to eliminate anomalies in a down-converted signal.